Replacements for bicondylar joints in human limbs

ABSTRACT

A surgical prosthesis or implant for replacing the natural components of a bicondylar joint in a human limb (leg or arm). Thus, a natural knee or elbow joint consists of two pairs of coacting load-bearing condyles. The prosthetic implant has two pairs of coacting male and female condylar components. The male and female components which replace the natural lateral condyles are spherical or spheroidal in shape to simulate a ball and socket joint, while the male and female components which replace the natural medial condyles are constituted by a rib or ridge and a groove. Both the rib and the groove are curved, in a plane normal to the general axis of the limb when straight or extended, to an arc of a circle centered on the center of articulation of the ball and socket, and are also curved in a plane parallel to the limb, the radius of the crest of the rib being less than the radius of the coacting base of the groove. Both male components and both female components can be formed on respective rigid carriers, or they may optionally be separate for individual fixation to the patient&#39;s limb.

This invention relates to replacements for bicondylar joints in humanlimbs and has for an object to provide an artificial replacement orprosthetic implant for the natural joint structure which simulates asnearly as is reasonably possible the true action of a healthy jointwhich, through disease or damage, has to be replaced.

A bicondylar joint is one in which the members of the limb arearticulated through two pairs of coacting male and female formationswhich serve both to carry the external loads normally imposed on thearticulation and to govern the action of the articulated limb membersduring flexing and extension movements. Considering, by way of example,a knee which articulates the tibia to the femur, this joint is not asimple hinge having a single fixed axis of rotation. In flexion andextension the tibia navigates a helical course on the lower end of thefemur, rotating outward on extension and inward on flexion. When thethigh muscles are relaxed, the tibia may be freely rotated on the femur,but when the muscles are contracted, and especially in weight bearing,there is synchronous rotation of the tibia on the femur, that is, thetibia always rotates outward when the knee joint extends, and inwardwhen it flexes.

The lower end of the femur is bicondylar in shape. The medial condyle islonger and curved, while the lateral condyle is rounded, and acts as aball-and-socket joint with the lateral condyle of the tibia. Thus, asthe knee straightens, the medial condyle of the tibia rotates outward onthe axis of the lateral, taking a longer course on the medial femoralcondyle. Thus, the tibia moves on the femur between the fully flexed andthe fully extended position in a generally helicoidal path under thecontrol of the muscles in the thigh. It is the action of these muscleswhich ensures the stability of the joint.

The elbow joint is similarly constructed except that the counterparts inthe forearm to the tibial condyles are formed separately on the radiusand the ulna, respectively, and these two bones moves independently whenthe forearm pronates and supinates.

The present invention is a prosthetic implant or replacement assemblyfor a bicondylar joint in a human limb (whether natural or artifical)having two pairs of male and female condylar components, the malecomponents comprising lateral and medial protruberances each engaging acomplementary lateral or medial female component such that duringflexion and extension of the joint, one of the two portions of the limbarticulated by the joint deflects relative to the other simultaneouslyabout axes in two planes at right angles, one plane lying substantiallyparallel to the general axis of the limb and the other lyingsubstantially normal thereto. Thus, for example, the normal helicoidalmotion of the tibia on the femur during flexion is reproduced by theimplant.

A preferred feature of the invention is that the male lateral condylarcomponent is a part-spherical or spheroidal protruberance and thecoacting female lateral condylar component is a part-spherical orspheroidal socket of closely similar dimensions.

According to a further feature of the invention, the male medialcondylar component is an arcuate rib or ridge having a curvature in eachof the said two planes, the curvature in the substantially normal planebeing centred at the common centre of the lateral components, and thecoacting female condylar component is a groove having the same radius ofcurvature in the said normal plane but a greater radius of curvaturethan that of the ridge in the plane which is substantially parallel tothe axis of the limb.

Alternatively, the contour of the base of the medial groove may beformed as an arc of a complex non-circular curve.

It is normally envisaged that the present invention will be applied to anatural complete limb (leg or arm) of a patient. In such a case, unlessthe entire joint has been damaged, as by widespread disease or completefracture, as much as possible of the original bone surrounding theactual coacting bearing surfaces will be preserved intact, and only thatpart of each limb member which, in the healthy condition, included thesesurfaces will be removed, leaving a cavity into which will be fittedsnugly a prosthetic implant or unit on which the replacement male orfemale surfaces are formed. This unit will then be secured in anyappropriate way -- as by adhesives or pegs -- in such a way that therepaired limb will function in the same way as the original healthylimb.

The invention is, however, also applicable to artificial limbs, in whichcase the male and female components of the replacement joint assemblymay be formed integrally with the respective artificial limb members.

A practical embodiment of the present invention will now be described byway of example only, as applied to a knee joint, with reference to theaccompanying drawings in which:

FIG. 1 is a diagrammatic representation of a flexed natural knee joint;

FIG. 2 is a composite under-plan view of the natural joint of FIG. 1showing the two limit positions of the tibia;

FIG. 3 is a schematic layout of the female components of a replacementassembly according to the present invention as seen in the plane whichis substantially normal to the general axis of a leg;

FIG. 4 is a part-sectional elevation on the line IV--IV of FIG. 3;

FIGS. 5 and 6 are plan views of the femoral and tibial condyles,respectively, of a replacement knee joint, the femur and tibia beingshown lying parallel to each other with their medial sides adjacent;

FIG. 7 is a part-sectional anterior elevation of a replacement rightknee joint in the extended or straight position;

FIG. 8 is a side elevation on the arrow VIII of FIG. 7;

FIG. 9 is a view similar to FIG. 7 showing the replacement joint in fullelevation at the limit position of full extension, and

FIG. 10 is a view similar to FIG. 9 showing the joint partly flexed.

Referring first to FIGS. 1 and 2, which are applicable to a right leg,the femur 10 and tibia 11 of a natural leg are articulated for rotationabout a generally horizontal mean axis represented by the line X--X. Theline A--A represents the axis of the tibia in flexion, and the line B--Brepresents the axis of the tibia in extension. These three axesintersect at a point 0 which represents the centre of a ball and socketjoint the ball part 12 of which constitutes the lateral femoral condylewhich works in a socket at 13 on the tibia.

The medial femoral condyle 14 partly rolls and partly glides on thecoacting medial tibial condyle part 15, so that, as the tibia 11 movesbetween the flexed position represented by the axis A--A and theextended position represented by the axis B--B in FIG. 1, it alsorotates on its longitudinal axis in the direction indicated by the arrowa in FIG. 1; whilst when it moves from the extended to the flexedposition it rotates in the reverse direction, as indicated by the arrowb in FIG. 1. The directions of rotation illustrated are applicable to aright leg, and will, of course, be opposite for a left leg. During theseflexing and extending movements of the tibia, the mean axis of rotationX--X is actually displaced angularly between limit positions shown atY--Y and Z--Z in FIG. 2. This figure shows, in full lines at 11a, theposition of the head of the flexed tibia relative to the bicondyloid endof the femur which is represented by the chain-dotted line 10; and indotted lines at 11b the position of the head of the extended tibiarelative to the femur. The figure thus illustrates how the mean axis ofrotation X--X of FIG. 1 actually displaces through an angle centred at 0from a flexed position Y--Y to an extended position Z--Z in FIG. 2.

While this angular displacement of the mean axis X--X of rotation of theknee joint has been taking place in a plane represented by FIG. 2 whichis substantially normal to the general axis of the complete leg, anotherangular displacement of the axis X--X takes place in a planesubstantially normal thereto--i.e. substantially parallel to the generalaxis of the complete limb. This is due to the motion of the femoralmedial condyle 14 on its counterpart surface 15 on the tibia 11, and asa result the tibia performs a helicoidal motion during flexion under thecontrol of the leg muscle.

In order to simulate this motion, and thus to avoid muscular strain ordistortion, when repairing a damaged bicondylar limb joint, counterpartmechanical components are made which, according to the presentinvention, cooperate to constrain the tibia to move through the samepath as that dictated by the natural condyles under the control of theirassociated muscles.

FIGS. 3 and 4 illustrate schematically the basic geometry of the kneejoint which leads to the helicoidal motion of the tibia during flexion.In FIG. 3, the lateral tibial condyle 13 is represented as apart-spherical depression, and the medial tibial condyle 15 as anarcuate groove having a radius R₁ in the plane normal to the tibialaxis. In FIG. 4, the medial tibial condyle 15 is shown as having aradius R₂ in a plane parallel to the tibial axis whilst the coactingfemoral medial condyle 14 has a lesser radius R₃ in the same plane. Theradius of the femoral medial condyle 14 in the plane of FIG. 3 is R₁ ;Thus, as the tibia 11 flexes on the femur 10, its medial condyle 15rolls in the planes of both FIGS. 3 and 4 while its lateral condyle 13twists on the femoral lateral condyle 12. In a natural joint the lateralcondyles 12, 13 tend to be spheroidal rather than purely spherical.

The invention embodies the above geometry in femoral (FIG. 5) and tibial(FIG. 6) replacement units 16, 17 respectively. In FIG. 5, the lateralcondyle 22 of the femoral replacement unit 16 is of prolate spheroidalform -- i.e. it forms a part of a sphere which is lengthened slightly inthe anterior-posterior direction. This condylar protruberance mates witha similarly prolate spheroidal tibial lateral condylar socket in thereplacement joint unit 17 of FIG. 6. The femoral medial condyle 24 ofthe replacement unit 16 (FIG. 5) is a curved rib which fits snugly, inthe transverse direction, into a tibial medial condylar groove 25 (FIG.6) in the replacement unit 17 in which the femoral condyle 24 is free toroll in the anterior-posterior direction.

One of the replacement units 16, 17 is preferably of metal and the otheris preferably of a low-friction synthetic resin plastic material. Asshown in FIG. 7, the femoral replacement unit 16 is of solid metalwhilst the tibial replacement unit consists of a metal base or carrier18 which is coated or lined with a layer of low-friction synthetic resinplastic material 19. Each unit 16, 17 has an integral rigid fixing pegor spike 20, 21 respectively for insertion, in conventional manner, intothe bone to secure the respective implant firmly and rigidly inposition. The size and shape of this peg or spike 20 or 21 is optional,and may be straight, curved, or shaped like a corkscrew to improve thegrip normally provided by a cement or like fixative.

As seen in FIGS. 7-10, the femoral replacement unit 16 or implant mayhave an integral upward anterior extension 26 the outer face of which isshaped at 27 to simulate the trochlear groove which, in a healthy knee,locates the patella 28 (FIG. 5) which may itself be natural orartificial.

The two views of FIGS. 9 and 10 indicate the helicoidal excursion of thetibia relative to the femur during flexion with a replacement jointaccording to the present invention. In FIG. 9, the units or implants 16,17 are shown in the extended position of the tibia. The femoral lateraland medial condyles 22, 24, respectively, nest in their respectivecoacting lateral and medial condyles 23, 25, the pegs 20, 21 beingsubstantially parallel. The femoral medial condyle 24 rests on the baseof the corresponding tibial medial condyle 25 at or near the anteriorend of the latter. As the tibia flexes, the zone of contact between thereplacement femoral and the tibial medial condyles moves posteriorlywhile that between the replacement lateral condyles 22, 23 remains insubstantially the same position. Consequently, as seen in FIG. 10, thereplacement tibial unit 17 both rotates and rolls outwards in ahelicoidal path which is substantially identical with that of thenatural knee joint.

The deepness of the tibial condyles 23, 25 and the snugness of their fitto the respective femoral condyles 22, 24 can be chosen at willaccording to the pathological condition of the particular limb undertreatment, and contribute significantly to the stability of thereplacement joint.

In a modification of the construction illustrated in FIG. 4 of thedrawings, the radius R₂ is not constant over the full length of thetibial medial condylar groove 15, the base of the latter being an arc ofa complex curve having a somewhat straighter or flatter intermediatesection. The contour of the groove 25 in the tibial replacement unit 17will then be correspondingly modified as required to restore themodified natural helicoidal motion of the tibia.

When the invention is to be applied to an elbow joint, the male condylesare shaped similarly to the femoral condyles 22 24 on a commonprosthetic implant or unit to be secured to the humerus, but the femalecondyles, which are shaped similarly to the tibial condyles 23, 25 areseparately mounted on the radius and the ulna respectively.

Although in the foregoing description the replacement male condyles 22,24 have been described as femoral and the coacting female condyleformations 23, 25 as tibial, it is to be understood that thisarrangement may, if preferred, be reversed so that the replacement malecondyles are secured to the tibia and the coacting replacement femalecondyles are secured to the femur.

Furthermore, the replacement condyles on either the femur or the tibia,or on both, may be separate implants, or may be interconnected by anarrow bridge on the anterior or posterior side of the joint. Suchmodifications may be of advantage if it is desired to avoid detachmentof the stabilizing ligaments.

I claim:
 1. A replacement assembly for a bicondylar joint articulatingtwo members of a human limb comprisingtwo pairs of coacting male andfemale load-bearing condylar components, the male components consistingof one spherical or spheroidal protruberance and one arcuate ridgehaving a curvature in each of two planes at right angles, one of saidplanes being normal to the general axis of the limb when straight andthe other being parallel to the said axis, the female componentsconsisting of one spherical or spheroidal socket adapted to coact withthe correspondingly shaped male component, and one arcuate groove beinga curvature in each of the said two planes, the curvature of the arcuateridge and the arcuate groove in the plane normal to the general axis ofthe limb being an arc of a circle struck about the centre of the sphererepresented by the spherical or spheroidal components.
 2. A replacementassembly as claimed in claim 1 wherein the curvature of the crest of theridge in the plane parallel to the limb is less than that of the base ofthe arcuate groove in the same plane.
 3. A replacement assembly asclaimed in claim 1 wherein the male components are formed integrally ona common carrier, and the carrier is adapted to be secured to the limb.4. A replacement assembly as claimed in claim 1 wherein the femalecomponents are formed integrally on a common carrier, and the carrier isadapted to be secured to the limb.
 5. A replacement assembly as claimedin claim 1 wherein the components representing the condyles on one halfof the joint are of metal and the coacting components are at leastcoated with a low-friction synthetic plastic material.
 6. A replacementknee joint assembly comprising lateral and medial pairs of coactingcondylar components, the lateral pair comprising a male femoralcomponent in the form of a spherical or spheroidal protruberance and afemale tibial component in the form of a coacting spherical orspheroidal socket, and the medial pair comprising a male femoralcomponent in the form of a ridge having a curvature in a plane normal tothe axis of the femur and a curvature in a plane parallel to the axis ofthe femur and a female tibial component in the form of a coacting groovehaving a curvature in a plane normal to the axis of the tibia and acurvature in a plane parallel to the axis of the tibia, the curvaturesin the planes normal to the axes of the femur and the tibia being arcsof circles of the same radius struck about the effective centre ofrotation of the lateral condylar components.
 7. A replacement knee jointassembly as claimed in claim 6 having integral fermoral condylarcomponents united on the anterior side by a forward and upward curvedtrochlear extension the anterior surface of which has a shallow groovefor locating the patella.
 8. A replacement elbow joint as claimed inclaim 1 having integral male condylar components for fixation to thehumerus and separate female condylar components for fixation to theradius and ulna respectively. .Iadd.
 9. A replacement assembly for abicondylar joint articulating two members of a human limb comprising twopairs of coacting male and female load-bearing mechanical condylarcomponents, the male components consisting of one spherical orspheroidal protruberance and one arcuate ridge having a curvature ineach of two planes at right angles, one of said planes being normal tothe general axis of the limb when straight and the other being parallelto the said axis, one of the female components being provided with aspherical or spheroidal socket adapted to coact with the correspondinglyshaped male component and the other of the female components beingprovided with one arcuate groove having a curvature in at least one ofthe said two planes, the curvature of the arcuate ridge and the arcuategroove in the plane normal to the general axis of the limb being an arcof a circle struck about the center of the sphere represented by thespherical or spheroidal components. .Iaddend. .Iadd.
 10. A replacementassembly as claimed in claim 9 wherein said at least one of the said twoplanes is the plane normal to the general axis of the limb..Iaddend..Iadd.
 11. A replacement assembly as claimed in claim 9comprising a common carrier supporting the male components, the carrierbeing adapted to be secured to one of the members of the limb..Iaddend..Iadd.
 12. A replacement assembly as claimed in claim 1comprising a common carrier supporting the female components, thecarrier being adapted to be secured to one of the members of the limb..Iaddend..Iadd.
 13. A replacement assembly as claimed in claim 9 whereinone pair of said components is of metal and the coacting pair ofcomponents includes a lining of a low-friction synthetic plasticmaterial. .Iaddend. .Iadd.
 14. A replacement elbow joint as claimed inclaim 9 having integral male condylar components for fixation to thehumerus and separate female condylar components for fixation to theradius and ulna respectively. .Iaddend..Iadd.
 15. A replacement kneejoint assembly comprising lateral and medial pairs of coactingmechanical condylar components, the lateral pair comprising a malefemoral component in the form of a spherical or spheroidal protruberanceand a female tibial component in the form of a coacting spherical orspheroidal socket, and the medial pair comprising a male femoralcomponent in the form of a ridge having a curvature in a planeintersecting the axis of the femur and a curvature in a second planeintersecting the first said plane and extending in generally the samedirection as the axis of the femur and a female tibial component in theform of a coacting groove having a curvature in a plane intersecting theaxis of the tibia, the curvatures in the planes intersecting the axes ofthe femur and the tibia being arcs of circles of the same radiusconcentric with the effective center of rotation of the lateral condylarcomponents. .Iaddend..Iadd.
 16. A replacement knee joint assembly asclaimed in claim 15 comprising a forward and upward curved trochlearextension, said femoral condylar components being united on the anteriorside by said trochlear extension the anterior surface of which has ashallow groove for locating the patella. .Iaddend.